Secondary recovery composition comprising water, hydrocarbon solvent and colloidal silica

ABSTRACT

A method of secondary recovery of hydrocarbons which involves injecting a fluid into an injection well and producing hydrocarbons from a production well involves the use of a novel thickened injection fluid. The fluid comprises an intimate mixture of water, hydrocarbon solvent and colloidal silica. The fluid is miscible with hydrocarbons in the reservoir and aqueous fluids and provides a decreased mobility ratio to enhance sweep efficiency.

llnited States Patent Christopher et a1.

[4 1 Jan. 21, 1975 Tex.; Jack H. Kolaian, Wappingers Falls, NY.

Assignee: Texaco Inc., New York, NY.

Filed: July 13, 1973 Appl. No.: 379,152

Related US Application Data Division of Ser. No. 239,388, March 29,1972, Pat. No. 3,780,808.

US. Cl 252/8.55 D Int. Cl E2lb 43/22 Field of Search 252/855 D; 166/275References Cited UNITED STATES PATENTS 1/1960 Meadors 166/275 6/1967Corrin 166/274 X 9/1965 Meadors 166/274 3,368,620 2/1968 Harvey i i166/274 3,377,275 4/1968 Michalski 252/855 D 3,412,792 11/1968 Parker eta1. 252/855 D 3,446,282 5/1969 Cooke, Jr 252/855 D 3,493,051 2/1970Gogarty 252/855 D 3,515,216 6/1970 Gies 1. 166/292 X 3,610,339 10/1971Harvey.... 252/855 D X 3,691,072 9/1972 HOlm 252/355 D 3,753,904 8/1973Holm 1. 252/855 D Primary Examiner-Benjamin R. Padgett AssistantExaminer-R. E. Schafer Attorney, Agent, or Firm-T. l-l. Whaley; C. G.Ries [57] ABSTRACT A method of secondary recovery of hydrocarbons whichinvolves injecting a fluid into an injection well and producinghydrocarbons from a production well involves the use of a novelthickened injection fluid. The fluid comprises an intimate mixture ofwater, hydrocarbon solvent and colloidal silica. The fluid is misciblewith hydrocarbons in the reservoir and aqueous fluids and provides adecreased mobility ratio to enhance sweep efficiency.

2 Claims, N0 Drawings SECONDARY RECOVERY COMPOSITION COMPRISING WATER,I-IYDROCARBON SOLVENT AND COLLOIDAL SILICA This is a division, ofapplication Ser. No. 239,388, filed Mar. 29, 1972, now US. Pat. No.3,780,808.

BACKGROUND. OF THE INVENTION In a hydrocarbon reservoir which has beenproduced to the point that it is devoid of natural energy; it may bedesirable to institute secondary recovery methods to recover the largeamount of hydrocarbon still left in the reservoir. Most secondaryrecovery operations are generally carried out by injecting an extraneousfluid into the reservoir which will then migrate to an output wellpushing the hydrocarbon ahead of it. The hydrocarbon is recovered at theoutput well. Even in secondary recovery operations about one-half of thehydrocarbon is left behind in the reservoir. The inefficiency of thedisplacement process is due to two retentive forces, viscosity andcapillarity. The retentive force of viscosity may be removed by heatingthe formation to a point where the viscosity of the reservoirhydrocarbon be comes equal to or less than the viscosity of thedisplacing fluid or by increasing the viscosity of the displacing fluid.However, if the displacing fluid is not miscible with the hydrocarbon,the retentive force of capillarity will not be removed. To remove theretentive force of capillarity it is necessary to use as a displacingfluid a material which is miscible with the hydrocarbon. If thedisplacing fluid is miscible with the reservoir hydrocarbon theinterface between the hydrocarbon and displacing fluid will be removedand, therefore, so will the retentive force of capillarity.

Displacement efficiency is a term referring to the amount of hydrocarbonremoved from the portion of the reservoir actually swept by thedisplacing fluid. Displacement efficiency may be low due to high surfacetension at the interface between the displacing fluid and thehydrocarbon in the reservoir. If this surface tension can be removed thecapillary forces will be reduced to zero and the hydrocarbon may becompletely displaced from the portions of the reservoir contacted by thedisplacement fluid.

Sweep efficiency is a term referring to the percentage of the reservoiractually contacted or swept by the displacing fluid regardless of theamount of hydrocarbon removed from the swept portion or displacementefficiency referred to above. A major cause of poor sweep efficiency isassociated with the fact that the injected displacement fluid generallyhas a lower viscosity than the hydrocarbon to be displaced.

If the viscosity of the fluid displacing the reservoir hydrocarbon tothe production wells is lower than the reservoir hydrocarbon, prematurebreakthrough of the driving fluid into the production wells will occur.The displacing fluid actually fingers through the reservoir hydrocarbonand proceeds to the production well before an adequate portion of thereservoir has been swept. The effects of viscosity on sweep efficiencymay be described in terms of the mobility ratio. The mobility ratio isdefined by the following equation:

where M mobility ratio u u, viscosity of displacing fluid and displacedfluid (hydrocarbon), respectively:

K K relative permeability of the formation with respect to thedisplacing fluid and the displaced fluid respectively.

At high mobility ratios the phenomenon commonly known as fingeringoccurs and the displacing fluid does not display a flat front to thereservoir hydrocarbon, but instead, rushes ahead at various points infinger like protrusions which may prematurely break through to theproduction wells. The hydrocarbon in areas not touched by the fingers ofdisplacing fluid is usually left unrecovered in pockets in thereservoir. Since most displacing fluids are more mobile than thedisplaced.

fluid, the hydrocarbon, the mobility ratio will usually be quite high,and a poor areal sweep efficiency will occur because of fingering.

As stated in the Reservoir Engineering Manual by Frank W. Cole, GulfPublishing Co, 1969, at page 230:

The capillary forces holding the oil in the reservoir rocks can beeliminated if an injection fluid is used which is miscible with thereservoir oil.

Although these miscible fluids will displace of the oil which theycontact, recovery is actually substantially less because of the lowviscosity and low density of the injected fluid. The low viscositycauses channel ing and bypassing, and the low density promotes gravitysegregation and consequent over-running of the oil. Because of these twofactors, this method works best in low viscosity, high API gravity oilreservoirs.

Polymeric compounds which increase the viscosity of the displacing fluidso as to lower the mobility ratio and increase the sweep efficiency ofthe displacing fluid have been developed and used in recent years. Forexample, US. Pat. No. 3,039,529 discloses the use of polyacrylamidesolutions to increase the viscosity of the displacing fluid. Also, US.Pat. No. 3,58l,824 discloses the use of polysaccharides for the samepurpose. Although these polymers are useful for increasing the viscosityof the displacing fluid they are expensive. Also, the displacing fluidcontaining these polymers tends to decrease in viscosity as it travelsthrough the reservoir away from the injection well bore due toabsorption of the polymer from solution and to mechanical degradation ofthe polymer.

The method of this invention provides an injection fluid which ismiscible with the reservoir hydrocarbon while having a high enoughviscosity so that the mobility ratio will be low enough to preventlingering.

The method of our invention provides an injection or displacing fluidwhich will not decrease in viscosity as it moves away from the injectionwell bore.

The method of our invention provides an injection fluid which ismiscible with both water and hydrocarbon.

The term reservoir hydrocarbon and the like in this disclosure refergenerally to oil of various viscosities found in subterraneanreservoirs.

SUMMARY OF THE INVENTION The invention is a method for recoveringhydrocarbon from subterranean hydrocarbon reservoirs which is penetratedby at least one injection well and one pro duction well wherein athickened fluid is injected into an injection well and hydrocarbon isproduced from a production well. The fluid comprises an intimate mixtureof water, a hydrocarbon solvent miscible with reservoir hydrocarbon andcolloidal silica. The invention is also the novel injection fluiddescribed above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS THE FLUID The injection fluiduseful in the process of our invention comprises an intimate mixture ofwater, a hydrocarbon miscible with the reservoir hydrocarbon andcolloidal silica. A surfactant and/or a polar multifunction compound maybe added to adjust the viscosity of the fluid.

The water may be fresh or mineral ladened as, for example, salt waterobtained from a subterranean formation in the vicinity of oil bearingzones. The water selected should be compatible with the formation it isto be injected into so that harmful swelling, for instance, will notoccur.

The colloidal silica useful in our invention is different fromprecipitated silica or silica gel. The colloidal silica useful in ourinvention is a fumed silica which is made up of chain-like formationssintered together. These chains are branched and have enormous externalsurface areas of from about 50 to about 400 meters /gram. Each segmentin the chain has many hydroxyl (OH) groups attached to silicon atoms atthe surface. When the segments come into proximity to each other, thesehydroxy groups will bond to each other by hydrogen bonding to form athree dimensional network.

The colloidal silica acceptable for use in the method of this inventionshould have a particle size ranging from about 7 to millimicrons (mu).In this size range the colloidal silica will pass through evenreservoirs with very small pore size. For example, a reservoir havingvery low permeability of say 0.016 millidarcies (md) has acorrespondingly small pore size of 25 to 100 mu. Thus, the colloidalsilica suitable for use in the process of this invention will passthrough even the small est pores encountered in hydrocarbon reservoirsand will maintain a constant viscosity in the displacing fluid.

Colloidal silicas are readily available from manufacturers. One sourceis the Cabot Corporation of Boston, Mass. under the trade nameCAB-O-SIL. Colloidal silica is also available from other commercialsources and the reference to one source is not intended to limit thescope of our invention.

When the silica particles are dispersed in a liquid medium, the networkstructure formed by the silica particles restricts the movement of themoelcules of the liquid medium. This results in an increase in theviscosity of the liquid.

The thickening efficiency of the silica is directly related to thepolarity of the liquid to be thickened. The use of selected additives(surfactants and/or multifunctional compounds) can increase thethickening efficiency of the silica. In the case of the hydrocarbonsolvent mentioned above, these additives react with the interfacebetween the silica and the solvent and increase the degree to which thesilica particles form the three dimensional network. This allows lesssilica to be used to achieve equivalent thickening of the solvent.Usually less than 0.5% of the additive based on the weight of totalliquid to be thickened will achieve marked increases in viscosity.

For liquids of high polarity such as water, aldehydes, ketones, ete.,cationic and nonionic surfactants, e.g., oleoyl trimethylene diamine andnonyl polyethoxy ethanols can cause dramatic increases in viscosity. Low

polarity or non-polar liquids, such as hydrocarbons, are

thickened by the use of anionic surfactants such as sodium linearallylate sulfonate and multifunctional com- 5 pounds such as ethyleneglycol.

Depending on the system, dramatic changes in viscosity can occur byusing two additives such as a nonionic surfactant, and a cationic type.

The hydrocarbon component of the injection fluid of our invention mustbe a solvent for the reservoir hydrocarbon, that is, it must be misciblewith the reservoir hydrocarbon. Examples of suitable hydrocarbons arearomatics such as benzene and toluene and aliphatics such as LPG,propane, butane, isobutane, pentane, isopentane, and hexane. Also, anymixture of suitable hydrocarbon solvents which when mixed retain theirmiscibility characteristics with the reservoir hydrocarbon areacceptable.

The multifunctional compounds mentioned above fall into the generalclass of compounds with a plurality of groups available for hydrogenbonding. Examples of such compounds are amines, ethylene glycol,glycerine, and proplyene glycol.

The surfactant which may be used in the injection fluid of our inventionmay broadly be any compound which reduces surface tension of the water,thus reducing the surface tension between the water and the reservoiroil. Soap may be used, for instance, the sodium salts of high molecularweight alkyl sulfates or sulfonates. Also very useful are nonionicsurfactants which are usually a reaction product of a hydrophobic and ahydrophilic material, such as the reaction product between alkyl phenolsand ethylene oxide.

The method of preparation of the injection fluid of our inventioninvolves blending. The technique described below has been found to forma satisfactory colloidal fluid of a specific viscosity. Other techniquesmay possibly be discovered which will also form a satisfactory fluid.The method used is given to aid in carrying out our invention and is notintended to limit the scope of our invention.

An example of the preparation of a typical fluid of our invention is asfollows:

1. Measure out 200 milliliter (ml) n-hexane into a Waring blender.

2. With blender at low speed, add 4 grams colloidal silica.

3. Blend 1 minute at 16,000 revolutions per minute (rpm) a soft gelforms.

4. Add with blending 3 ml of a nonionic surfactant,.

3 ml glycerine, and 20 ml water a firm gel forms. The gel so formed, ifprotected from evaporation of the water and hexane is stable and couldbe transported to the well site as is.

5. At least 1,000 ml more hexane and 210 ml water are added to the firmgel to reduce its viscosity to that to be used for injection.

A material made, as outlined above, has remained a stable liquid ofabout centipoise viscosity for a year.

The fluid of our invention should be adjusted in viscosty so that themobility ratio is not less than about 0.1 nor more than about 10. At thehigher mobility ratios fingering will have more of a tendency to occurand at the lower mobility ratios the fluid will become progressivelymore viscous and difficult to pump. It is espe cially preferred that themobility ratio range from about 0.9 to about 3.

Of course, once the desired mobility ratio is known, the necessaryviscosity of the fluid may easily be calcu lated. The viscosity of thefluid described in our invention may be tailored to fit the needs of theuser by variations of ingredients. Due to the number of ingredients,

a detailed explanation of methods of varying viscosity is impractical togive. However, it will be evident to one skilled in the art what effecteach ingredient has on the viscosity so that an infinite number offluids may be made which will fall within the scope of our invention.USE OF THE FLUID A firm gel of the hydrocarbon solvent to be used willbe prepared containing the hydrocarbon, water, surfactant, polarmultifunctional additive and colloidal silica. Ifa less firm gel isdesired the surfactant and multifunctional additives may be reduced oreliminated. The techniques of preparation as described above may beused. This gel may then be reduced to the proper viscosity with water,which thickens the fluid to a point due to hydrogen bonding, above whichadditional water thins it. The amounts of water to be used will dependon the amounts of the other ingredients and the ingredients themselves.The gel may also be reduced in viscosity by adding additionalhydrocarbon solvent.

The fluid of this invention is then injected into the reservoir in orderto displace the oil in the reservoir to production wells. The fulid ofour invention may conceivably be the only fluid injected into theformation but considering its relatively expensive character and thevast quantities which would be needed to flood an entire oil reservoir,it is preferred to use the fluid of our invention as a slug. A slug ofthe fluid of our invention would be injected into the reservoir followedby another fluid. The trailing fluid may be water, gas or some treatedfluid.

The thickened miscible slug of our invention may be used according tostandard methods of miscible slug displacement. The lower mobility ofthe miscible slug of our invention will remove problems of fingering andoverriding encountered with conventional miscible fluids. It is withinthe skill of the art to determine the proper slug size and rate ofdisplacement to be used. As is known by those skilled in the art thereis considerable controversy over the size of miscible slug which shouldbe used. A standard text: Mechanics of Secondary Oil Recovery, Smith,Reinhold, New York, 1966, provides much basic information on miscibledisplacement.

One advantage to using the fluid of this invention is that while it ismiscible with oil at the leading edge of the slug; it is also misciblewith the fluid which follows it such as water. This double miscibilityresults in a gradual viscosity gradient at the trailing edge of thefluid slug of our invention. This viscosity gradient prevents a sharpviscosity difference and, therefore, there is little tendency for thetrailing fluid to finger into the slug.

The fluid of this invention also provides an improvement over thepolymer thickened fluids in shear resistance. When polymer thickenedfluids are subjected to the tremendous shear forces present as they arepushed through the reservoir rock they lose viscosity, particularly inthe vicinity of the well bore, and the mobility ratio rise results inpossible fingering effects. However, the fluid of our invention does notlose appreciable vis cosity due to shear forces.

Injection of the fluid of this invention may be in a secondary recoveryoperation or in a tertiary recovery operation. For example, after aconventional water flood or polymer flood or any other secondaryrecovery operation the fluid of this invention may be injected to removeadditional hydrocarbons.

EXPERIMENTAL A linear, unconsolidated sand pack was saturated with 69.2ml of Brelum crude (Duval County, Texas, viscosity 38 cp). The packcontained no connate water. It was waterflooded at constant pressuredrop with 2.31 pore volumes of 2% NaCl solution and 34,4 cc (49.7%) ofoil were produced. The pack was then flooded with 1.5 pore volumes of250 ppm polyacrylamide solution in 2% NaCl and an additional 16.1 ml ofoil (23.3%) were produced. This was followed by 2.7 pore volumes of 2%NaCl which produced an additional 1.0 ml of oil (1.5%). Thus a total of51.5 ml (74.5%) ofoil had been produced. This left the pack containing17.7 ml of oil, a saturation of 25.5%.

The fluid described on page 8 of the application was injected into thepack in a tapered slug. Approximately 10.8 ml (15.6% pore volume) wasinjected directly, followed by 10 ml blended into the leading edge of aslug of 250 ppm polyacrylamide in 2% NaCl. Approximately 25 ml (36%P.V.) of polyacrylamide slug was injected, blended into 2% NaCl whichwas injected last. The 2% NaCl slug comprised approximately one porevolume. At this point 60.5 ml (87.5%) of the oil had been recovered.Thus the thickened hydrocarbon slug and its drive liquids resulted inthe recovery of an additional 13% recovery from a sand pack that had hadextensive previous waterflooding and in fact had been polymer flooded.

It appeared that, in this pack containing only 25.5% oil saturation, oilwas banked and moved ahead of a clean sand zone as the flood progressed.

We claim:

1. A fluid comprising about 64 weight percent hexane, 35 weight percentwater, 0.3 weight percent colloidal silica, 0.3 weight percent nonionicsurfactant and 0.3 weight percent glycerine.

2. A fluid as in claim 1 made by mixing one sixth of the total hexanewith the colloidal silica until a soft gel forms,

adding the glycerine, the nonionic surfactant and about 91 percent ofthe water and mixing until a firm gel forms, and

adding the remainder of the hexane and water until a liquid forms.

2. A fluid as in claim 1 made by mixing one sixth of the total hexane with the colloidal silica until a soft gel forms, adding the glycerine, the nonionic surfactant and about 91 percent of the water and mixing until a firm gel forms, and adding the remainder of the hexane and water until a liquid forms. 